We have an 8am CP res in May. We are also planning to hit some non RD necessary rides that we missed on our first trip to MK, like Jungle Cruise and Pirates, before FPs start. I'd like to get out of breakfast before rope drop, but it's not a huge rush for us. Yes, Main street is not completely empty, but it's just a fun and unique experience to be the first people in the park. I learned my lesson to check in at CP right away before taking pictures in front of the castle. The check in line gets really long if you don't head there first. Not a big deal, but makes kids a little antsy.
We previously took castle pics with NO one else on Main St in the shot. However, there was a big barrier wall around the castle and a crane sticking out. So there may never be a "perfect" pic.